The metal-oxide-semiconductor (MOS) structure, commonly referred to as the MOS capacitor, is a two-terminal device with one electrode connected to the metal and the other electrode connected to the semiconductor, forming a voltage-dependent capacitor. The acronym MOS is used even if the top electrode is not a metal and the insulator is not an oxide. In a MOS capacitor system, the inversion layer is formed by thermally generated minority carriers. The concentration of minority carriers can change only as fast as carriers can be generated within the depletion region near the surface. The basic operation of an MOS capacitor system is discussed at equilibrium and under biasing conditions. Analytical model of MOS capacitor system is developed to discuss the accumulation, depletion, and inversion mode operations of MOS capacitor structures.